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1. Time-domain Fabry–Perot resonators formed inside a dispersive medium

   https://doi.org/10.1364/JOSAB.428411  

   Zhang, J.; Donaldson, W_R; Agrawal, Govind_P (July 2021, Journal of the Optical Society of America B)

   We show that the temporal analog of a Fabry–Perot resonator (FPR) can be realized by using two moving temporal boundaries, formed by intense pump pulses inside a dispersive medium (such as an optical fiber). We analyze such FPRs using a transfer-matrix method, similar to that used for spatial structures containing multiple thin films. We consider a temporal slab formed using a single square-shape pump pulse and find that the resonance of such an FPR has transmission peaks whose quality ( $Q$) factors decrease rapidly with an increasing velocity difference between the pump and probe pulses. We propose an improved design by using two pump pulses. We apply our transfer-matrix method to this design and show considerable improvement in the $Q$factors of various peaks. We also show that such FPRs can be realized in practice by using two short pump pulses that propagate as solitons inside a fiber. We verified the results of the transfer-matrix method by directly solving the pulse propagation equation with the split-step Fourier method. 

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2. Time-dependent growth of nuclear spin polarization under periodic optical pumping

   https://doi.org/10.1103/PhysRevB.108.115203  

   Dominguez, Michael J.; Hsu, Hua-Wei; Sih, Vanessa (September 2023, Physical Review B)

   Consecutive circularly-polarized optical pulses generate and rotate electron spin polarization through optical orientation and the optical Stark effect. We perform time- and magnetic-field-dependent optical pump-probe measurements on gallium arsenide and observe a variable Overhauser field growth that depends on the external magnetic field and laser wavelength. We show that the time dependence of the nuclear spin polarization can be attributed to the time-averaged electron spin polarization produced along the external magnetic field direction. 

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3. Large effective magnetic fields from chiral phonons in rare-earth halides

   https://doi.org/10.1126/science.adi9601  

   Luo, Jiaming; Lin, Tong; Zhang, Junjie; Chen, Xiaotong; Blackert, Elizabeth R; Xu, Rui; Yakobson, Boris I; Zhu, Hanyu (November 2023, Science)

   Time-reversal symmetry (TRS) is pivotal for materials’ optical, magnetic, topological, and transport properties. Chiral phonons, characterized by atoms rotating unidirectionally around their equilibrium positions, generate dynamic lattice structures that break TRS. Here, we report that coherent chiral phonons, driven by circularly polarized terahertz light pulses, polarize the paramagnetic spins in cerium fluoride in a manner similar to that of a quasi-static magnetic field on the order of 1 tesla. Through time-resolved Faraday rotation and Kerr ellipticity, we found that the transient magnetization is only excited by pulses resonant with phonons, proportional to the angular momentum of the phonons, and growing with magnetic susceptibility at cryogenic temperatures. The observation quantitatively agrees with our spin-phonon coupling model and may enable new routes to investigating ultrafast magnetism, energy-efficient spintronics, and nonequilibrium phases of matter with broken TRS. 

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4. Temperature-resilient random number generation with stochastic actuated magnetic tunnel junction devices

   https://doi.org/10.1063/5.0186810  

   Rehm, Laura; Morshed, Md Golam; Misra, Shashank; Shukla, Ankit; Rakheja, Shaloo; Pinarbasi, Mustafa; Ghosh, Avik W.; Kent, Andrew D. (January 2024, Applied Physics Letters)

   Nanoscale magnetic tunnel junction (MTJ) devices can efficiently convert thermal energy in the environment into random bitstreams for computational modeling and cryptography. We recently showed that perpendicular MTJs actuated by nanosecond pulses can generate true random numbers at high data rates. Here, we explore the dependence of probability bias—the deviations from equal probability (50/50) 0/1 bit outcomes—of such devices on temperature, pulse amplitude, and duration. Our experimental results and device model demonstrate that operation with nanosecond pulses in the ballistic limit minimizes variation of probability bias with temperature to be far lower than that of devices operated with longer-duration pulses. Furthermore, operation in the short-pulse limit reduces the bias variation with pulse amplitude while rendering the device more sensitive to pulse duration. These results are significant for designing true random number generator MTJ circuits and establishing operating conditions. 

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5. Amplification of elliptically polarized sub-femtosecond pulses in neon-like X-ray laser modulated by an IR field

   https://doi.org/10.1038/s41598-022-09701-z  

   Khairulin, I. R.; Antonov, V. A.; Ryabikin, M. Yu.; Berrill, M. A.; Shlyaptsev, V. N.; Rocca, J. J.; Kocharovskaya, Olga (April 2022, Scientific Reports)

   Abstract Amplification of attosecond pulses produced via high harmonic generation is a formidable problem since none of the amplifiers can support the corresponding PHz bandwidth. Producing the well defined polarization state common for a set of harmonics required for formation of the circularly/elliptically polarized attosecond pulses (which are on demand for dynamical imaging and coherent control of the spin flip processes) is another big challenge. In this work we show how both problems can be tackled simultaneously on the basis of the same platform, namely, the plasma-based X-ray amplifier whose resonant transition frequency is modulated by an infrared field. 

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